There are a number of areas which benefit from having more accurate information concerning the types, distribution and characterization of forces applied to and by the human foot during human locomotion. Such information is relevant to the design and manufacture of shoes because of increasing sophistication in shoe design requiring a more clear indication of the types of forces applied, their distribution, and the shoe construction necessary in order to provide the proper design emphasis concerning factors such as force application, shock absorbtion, and energy distribution and dissipation concerns. This is particularly true in the design of high performance running footwear. The resiliency of the shoe and the manner of absorption and dissipation of energy have a significant effect upon the amount of energy that a runner must exert in order to travel at a certain speed. Shoes which have too high resiliency provide good shock absorbing capability and protect the foot but may also slow a runner because of the additional energy which is dissipated in the sole. However, other designs providing lower resiliency may minimize the amount of energy dissipation and produce more energy return to the human foot and hence improved speed or jumping capability.
Contact forces generated by human feet also are relevant to the design and construction of running surfaces such as indoor and outdoor track surfaces. As in shoe design, various track designs may differ in resiliency and other characteristics of the track surface. Improved techniques for gathering information on performance as a function of track resiliency, slope, surface smoothness or other parameters may facilitate the development of improved track surfaces and or better track materials and spacial configurations.
In addition to the mechanical aspects of the track and shoe, it may also be desirable to better understand the interaction action between the human foot structure and its interplay with the shoe and track in a coupled mechanical system so as to best design the shoe to accommodate the loading applied by the human foot. It is also desirable to create a match between the human foot and the shoe so that forces are applied in a manner which is attuned to the particular goal or balance of goals. For instance, in long distance running it may be most desirable to design a shoe so that human foot-shoe-ground interaction minimizes pressure points and alleviates the physical stress placed on the feet. Alternatively, it may be desirable in middle distance and sprint racing to have shoes which are designed to provide greatest speed capability with somewhat less concern about the stress level which may be applied by and to the foot since substantial mechanical injury to the skin and sole tissues of the foot are less likely to occur during relatively short periods of higher speed running.
Orthopedic surgeons and doctors are also in need of systems which provide relatively quantitative indications of stride variations, timing and associated foot contact pressures and forces. This information may be useful to the orthopedic professional for analysis of bone implants, joint ailments, muscular deficiencies, improper running or walking techniques and other medical or training related phenomena which can be studied, analyzed and/or inidicated as a result of information concerning foot contact forces, their timing and distribution.
Previously it has been common to employ force plates for determination of contact forces by a human foot. Force plates utilize various types of sensors which indicate the total vertical, and in some cases lateral forces, applied by the human foot as it contacts the force plate. Unfortunately, force plates have several disadvantages. The force plates are typically mounted in a fixed position and the human subject walks or runs over the force plates. Accordingly, it is necessary for the subject to properly space and time his or her stride so as to apply foot contact at an appropriate point on the force plate. This necessarily causes the subject to vary his or her stride to provide such results. This prevents unrestrained motion leads to force information, and any stride information, which is altered by the conscious effort of the subject to coordinate foot fall patterns and timing so as to comply with the force plate scheme. Such systems are not suitable for analyzing foot force and pressure information during free locomotion and under a variety of different gaits.
An insole has been developed for use in measuring stress distribution on the plantar surfaces of the human foot during walking and running by Hennig, Cavanaugh, Albert and MacMillan. The Hennig et al. device utilized 499 different piezoelectric ceramic transducers. The transducers each were approximately 4.78 millimeters square by 2 millimeters thick and constructed of lead zirconate titanate crystals imbedded in a 3 to 4 millimeter thick layer of highly resilient silicone rubber that apparently was molded about the 499 ceramic transducers to produce a flexible array which was designed to be impervious to moisture and electrically insulative between the different transducer elements. The transducers had silver electrodes which were diffusion bonded to their major surfaces. The transducers were laid out in a square array substantially covering the plantar surfaces of an insole. The insole was installed in a specially designed shoe and connected to a computer system via hardwired connections which severely limited mobility and prevented unrestrained motion from being analyzed.
The Hennig et al. force measurement system and transducer array did not provide means for evenly distributing the force applied across each individual transducer crystal. Instead variations of force due to varying elasticity and thicknesses of the tissue of the foot and insole produced contact stresses which were nonuniform over the surface of each individual transducer. Such variations in the applied stress or other causes reduced the charge output generated by the transducer thus leading to output signals which had relatively lower signal-to-noise ratios than are achievable with the present invention.
The Hennig et al. device also suffers from severe limitations in terms of the cost. Construction of insoles according to their teaching requires use of nearly 500 elements in one insole design and twice that amount for simultaneous monitoring of both feet, as required for stride analysis. The cost of construction of such devices is many thousands of dollars for each insole due to the numerous transducers used. The transducers are also embedded in a molded silicone rubber insole which necessarily is especially adapted to a particular foot size both with respect to the layout of the transducer array and the size of the silicone rubber insole itself. This necessitates construction of numerous different insoles in order to test a broad range of subjects. Because of the financial burden imposed by this type of construction, many research projects have not been able to utilize the Hennig system in testing of numerous different subjects having various shoe sizes.
Therefore, there remains a need for transducer assemblies and foot force detection systems which provide accurate foot force detection in an economical manner and can be adapted for sensing and recording information from a variety of different subjects without utilizing specially constructed shoes for each different foot size. There also remains a need for a system which is relatively portable and capable of measuring human foot contact forces during unrestrained motion.